In the conventional commercial process for the manufacture of nitric acid ammonia is oxidized by contact with air at elevated temperature over noble metal catalyst to form initially nitrogen oxide, which in the presence of excess oxygen is further oxidized to nitrogen dioxide or its dimer. The (NO.sub.2).sub.x is absorbed in water to produce nitric acid as illustrated by the equation: EQU 3 NO.sub.2 + H.sub.2 O .fwdarw. 2 HNO.sub.3 + NO (A)
The nitrogen oxide thus rleased is reoxidized to NO.sub.2 by contact with so called "bleach" air introduced into the absorber.
In the modern practice such plants are operated at superatmospheric pressure of seven atmospheres or higher to take advantage of increased oxidation rate of NO to NO.sub.2. The history of the development of the pressure process thus far described and the reaction mechanisms involved as well as certain of the calculations entering into the design of plants of this kind are reviewed in a monograph by T. H. Chilton, entitled "The Manufacture of Nitric Acid by the Oxidation of Ammonia"; Chemical Engineering Progress Monograph Series, No. 3, Vol. 56 (1969).
Since the several progresive reactions involved in the conversion of NH.sub.3 to HNO.sub.3 are exothermic the energy thus liberated is utilized to supply at least part of the power for compressing the air to the desired operating pressure. In a conventional commercial system, the tail gas from the absorber is reheated to the required temperature for operation of the expansion turbine system furnishing power for driving the air compressors.
With the application of stricter standards on fume abatement and to protect the turbine blades from corrosion by the tail gas, it has been the practice to purify the gas prior to introducing the same into the turbines or discharging to the atmosphere. This can be accomplished by passing the preheated tail gas over a noble metal catalyst in the presence of a reductant, such as a hydrocarbn fuel, which reduces the NO.sub.x in the tail gas to innocuous elemental nitrogen while residual oxygen in the gas stream is consumed by combustion of the hydrocarbons to form CO.sub.2 and water. Since additional sensible heat is thus released in the NO.sub.x abatement unit, the additional energy thus made available is beneficially utilized in supplying power for operation of the gas expansion turbines.
Although a substantial part of the thermal energy of the gas employed in driving the expansion turbine system is derived from exothermic process heat released in the oxidation of ammonia to nitic acid and that released in the catalytic NO.sub.x abatement unit, this heat content is generally insufficient in itself to meet the net power requirements of the turbine system in modern plants operating at superatmospheric pressure. Additional heat is generally supplied by direct heating of the tail gas in a burner to which external fuel is supplied together with air to support combustion. Such heating of the tail gas, moreover, raises the temperature thereof to an efficient level for promoting the catalytic reduction of the residual NO.sub.x in the abatement unit.
As seen from the foregoing description, the air compressors driven by the expansion turbines supply air at superatmospheric pressure utilized in the nitric acid plant. The stream of compressed air thus supplied may be divided into three individual branch streams, providing (1) reactant air furnishing oxygen for initial reaction with ammonia in the converter, (2) bleach air for oxidation of NO in the absorber, and (3) air utilized to support combustion in the direct fired heater.
To obtain the desired high nitric acid production rates at maximum efficiency, it is important not only that the flow of air and ammonia to the converter for the initial oxidation reaction be regulated but also that controls be maintained on the total air supplied to the system by the compressors. Even though a plant may have been initially designed for appropriate flow rates and system power balance, unintended variations in air flow which may result from changes in pressure and/or temperature of the incoming air supplied to the compressors, or intentional changes in production schedules will necessitate adjustment of the several components of the system to satisfy the new conditions imposed. Because of the interdependent relationships of the various components of the system, it will be appreciated that even small changes in any one of these, unless properly compensated, will throw the whole system out of balance and may "snowball" the effect of such change with consequent deleterious influence on the efficiency and economics of the plant operation.
Various concepts have been suggested or attempted for monitoring and controlling nitric acid plant operation, none of which have been found fully satisfactory to obtain the desired objectives. In modern plants a constant ratio of air to ammonia introduced into the oxidation converter is automatically maintained by provision of ratio set stations responsive to measured variations in air flow rate. To maintain the designed production rate, however, the flow of air to the ammonia converter must also be set and maintained substantially constant despite possible variation in the flow rate of air discharged by the compressor system. In preliminary studies leading to the present invention, it was found that in attempting to manipulate the firing rate in the direct fired heater to provide a controlled steady supply of air at the compressor outlet, there was a massive thermal inertial lag between the point at which the firing rate was changed and the point at which the effect of such change is ultimately felt.